(A) Field of the Invention
The present invention concerns a polyester resin composition having a high crystallization rate and excellent moldability upon using molds at low temperature, with less reduction of molecular weight upon preparing and molding the resin composition, and of satisfactory melt degradation stability and hydrolysis resistance.
(B) Description of the Prior Art
Thermoplastic polyesters have excellent mechanical properties, as well as physical and chemical natures and, accordingly, they have been used in wide application use. Among them, several thermoplastic polyesters, for example, polyethylene terephthalate have particularly excellent properties but they have not yet been used so generally at present for injection molding.
This is due to the fact that polyethylene terephthalate, in particular, involves a problem in moldability in that it has a slower crystallization rate as compared with other crystalline polymers and, therefore, is not crystallized sufficiently in a mold at a low temperature from 50.degree. to 95.degree. C. which is usually used for injection molding, and so it cannot give a satisfactory molded product.
In view of the above, with an aim of improving moldability, various studies have been made on additives that can improve the crystallization rate of thermoplastic polyesters. For instance, there have been known those compositions comprising thermoplastic polyesters to which an alkali metal salt of salicylic acid is added as disclosed in Japanese Patent Publication (Kokai) 61-157556, or an alkali metal salt of phenol sulfonic acid as disclosed in Japanese Patent Publication (Kokai) 56-57825.
However, in these conventional compositions described above, the crystallization rate of polyesters can not be improved much, and also the molecular weight of the polyesters is remarkably reduced upon melt molding or during production of resin compositions, and therefore only those molding products having poor mechanical properties are obtained. In addition there has been a problem that hydrolysis resistance of the polyester is deteriorated if an additive having a relatively great improvement of the crystallization rate is added.